(I should define this here because googling brings up mostly non-grammatical instances.) Term used in the CGEL for the phrase that can be equated to a dummy subject 'there', e.g.
There are three men in the garden.
In most cases it might have been the subject instead:
Three men are in the garden.
It has been displaced from subject position across the verb, but occurs before the complements and adjuncts ('in the garden'). This distinguishes it from the extraposed subject of a dummy 'it' clause, which is extraposed past the complements as well:
It is remarkable to me that men are in our garden.